Abstract
Body size is one of the most obvious and most important characteristic of any organism. A thorough understanding of how and why a certain individual obtains a specific body size, given its evolutionary history and ecological context, is a fundamental question in biology. One special case of variation in size
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is clinal variation: individuals of the same species indigenous to higher latitudes are on average larger than their conspecifics inhabiting regions at lower latitudes (closer to the equator). This pattern has a genetic basis, is very common in many animal species and represents a long-standing puzzle in evolutionary biology.
Drosophila melanogaster populations from temperate and tropical regions were known to produce the clinal variation in body size, very likely as a result of adaptation to the different climates. Aim of this study was to understand the evolution of this genetic difference in average body size between populations adapted to low and high temperatures. Analyses of correlated life-history traits, interaction of selection pressures, biochemistry of resource allocation and global gene expression resulted in formulating the following scenario. We suggest that geographical variation of adult body size in Drosophila melanogaster is plausibly the result of thermal evolution of a genetic trade-off between adult size and larval survival. Because a cold environment seems to select for a large adult body and a warm environment seems to select for increased larval vigor to resist challenges to pre-adult survival, alternative resource allocation strategies have evolved. Cold adapted genotypes seem to invest glycogen reserves preferentially in growth while warm adapted genotypes seem to invest their glycogen reserves preferentially in larval survival. The physiological basis of these alternative strategies is plausibly variation in the utilization of glycogen induced by variation in glycogen processing enzyme activities. A large fraction of the genes found to be differentially expressed across genotypes with different body size are involved in cell growth and maintenance. This suggests that the actual molecular determinants of the divergence in cellular metabolism and body size might be effectors and regulators of cell growth and differentiation through variation in the activity of signal transduction pathways.
Thus, the real difference between individuals from different geographical populations seems to be not a quantitative but a qualitative one. Warm and cold adapted genotypes are fundamentally different in their cellular metabolism and physiology. This underlying divergence allows them to exhibit alternative resource allocation strategies that result in adaptive life-histories
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